Display devices which are based on the use of so-called liquid crystals, during the recent years have found their application in many different fields. As examples there may be mentioned digital watches, different types of measuring instruments, and electronic applications in the provision of informative messages in the form of e.g. combinations of figures and letters. Such display devices of the liquid crystal display (LCD) type contain a number of liquid crystal cells or elements which are usually segment-oriented so as to form the actual characters, e.g. 7-segment or 16-segment alpha-numeric characters. As known, the liquid crystals are molecules which are capable of electric polarization, and a liquid crystal element is activated by the application of a suitable DC voltage, e.g. 5 volts, so that the liquid is polarized and changes from being transparent to a dark grey tone. When placing a polarization filter over a liquid crystal element at right angles to the polarization direction, a much darker tone appears when the element is activated, so that better contrast is achieved.
In the previously known liquid crystal displays one has, in accordance with the conventional technique, considered it to be necessary to use a non-transparent contrast foil at the rear side of the LCD unit in order to improve the contrast between the displayed characters and the surroundings. The contrast foil causes the LCD unit to be non-transparent also in the non-activated condition, and this represents a limitation in the possible applications. Thus, the contrast foil delimits the use to the display of "two-dimensional" information, i.e. information which is present in a plane such as is the case in the above-mentioned fields of application.
A device of the introductorily stated type is known from DE publication print No. 3 120 601. This known device is intended for use in "observation devices", vehicles and the like, and comprises a liquid crystal display plate which, in use, can be placed in a viewer's field of view or is arranged in the marginal region thereof in such a manner that it does not interrupt the visual observation in the viewer's field of view. The display plate consists of a pair of parallel glass plates with a liquid crystal mixture and electrodes placed therebetween, and behind the display plate there is placed a means for illuminating the plate in dim light and in darkness. Further, the plate is also preferably covered by a partly reflecting foil to increase the contrast. Thus, the device is a quite visible unit also in non-activated condition, even if it is designed with a view not to disturb the visual observation in a viewer's field of view.